The invention relates generally to hermetically sealed packages for high frequency microelectronic circuits, and in particular to a high reliability package for microwave hybrid and integrated circuits.
The operating frequency for data processing and data communications circuits is rapidly increasing, from the KHz range to the GHz range. At high frequency, the parasitic capacitance and inductance of the package enclosing the hybrid circuit or IC becomes a major factor, and as a result, the package must be considered a part of the circuit. Also, component densities and heat dissipation requirements are increasing, thus requiring the package to be a good thermal conductor. In these high frequency applications, the package must be compatible with microstrip transmission lines, and surface mount packages are becoming the industry standard. In addition, for analog circuits operating above about 500 MHz, it is very important that the package provide a good RF ground.
Existing packages for microelectronic circuits provide for some but not all of these needs. Traditional chip carriers and ceramic and metal flat packs cannot operate at high frequency, do not provide good RF grounding, and can handle only low power applications. Metal packages, e.g., TO-8 or TO-12 types, are not readily adaptable for housing ICs, cannot be surface mounted, and have a high VSWR at microwave frequencies.
One prior package comes close to meeting all of the criteria required, but due to its parasitic inductance and capacitance, its performance deteriorates at frequencies above 5 GHz. The construction of the package also makes it relatively expensive. A cross sectional view of the construction of this package is shown in FIG. 1. The package comprises a base 11, a lid 13, leads 15 and a ground plane attachment pad 17. The top surface of the base 11 carries thin or thick film circuit elements 19 and components 21. The circuit elements and components are connected to the leads 15 by plated through holes 23 in the base. Other plated through holes, not shown, connect to the ground plane attachment pad 17 to provide an RF ground. The lid is brazed to the top surface of base 11 to make a hermetic seal 25 around the perimeter. The plated through holes, however, still must be sealed to hermetically seal the package. To do this, the leads and ground plane attachment pad must be carefully brazed to the bottom of the base. This requires precise process control and adds to the cost of assembly of the package. In addition, there is no good RF ground plane along the plated through holes so the VSWR of the package becomes high above about 3 GHz. Also, due to the general design of leads 15 and ground plane attachment pad 17, both need to be soldered to the PC board. Because leads 15 are mounted onto the bottom of base 11, the user cannot inspect the package for solder bridging between pad 17 and leads 15 after attachment to the PC board.
An object of the invention is to provide a hermetic package for microelectronic circuits such as microwave frequency ICs and hybrid circuits, which can be surface mounted, has low parasitic capacitance and low inductance to RF ground, lead impedance close to 50 ohms and good power dissipation.
Another object of the invention is to provide a hermetic package for microelectronic circuits that has low piece part and assembly costs and is adapted to automated assembly.
These and other advantages are provided according to the invention by a package comprising a ceramic base with conductive, hermetically sealed vias that connect the metal ground plane on the bottom surface of the base to the circuit on the top, a ceramic frame sealed to the top surface of the base, leads sealed between the frame and the base, and a lid sealed to the top of the frame.
The vias are cofired with the ceramic base and provide good power dissipation because of their thermal conductivity. The vias can be located at any position on the area of the base giving flexibility to the circuit designer by providing good RF grounds for the circuit and can be used to isolate different stages in the circuit and heat sink higher power dissipated components. Because the vias are hermetically sealed, the package is more cost effective. No separate bottom component and no brazing operation is required to hermetically seal the package. Thus, the construction essentially eliminates the "classical" package reducing piece part and assembly costs and eliminating the electrical transition from the package to the circuit.
The leads have a tapered section so the leads are wider outside the package reducing inductance and narrower going through the ceramic frame to reduce capacitance, thus tuning the leads to close to 50 ohm impedance. A layer of conducting silver loaded solder glass underneath the lead electrically connects the lead to the circuit and seals the lead to the base. A nonconducting layer of lead oxide solder glass over the lead hermetically seals the lead between the ceramic frame and the ceramic base. The top surface of the frame has a metallized layer so that the lid can be hermetically sealed to the frame by soldering.
Because the leads extend out of the side of the package, the user can inspect for solder bridging between the package base and the leads after the package has been soldered to the PC board.
This construction overcomes the problems of prior packages to provide a hermetic package which can be surface mounted, has low parasitic inductance to RF ground, 50 ohm lead impedance and good power dissipation.